Clinical features, findings of diagnostic studies, results of therapy, and prognostic factors were analyzed in 45 patients with brain abscesses. The number of patients diagnosed yearly has increased since CT scanning became available, but despite the enhanced sensitivity, the time from either onset of symptoms or hospital admission until initiation of therapy was not decreased and there was no dramatic effect upon morbidity or mortality in this series. Infections of paranasal sinuses, ears, lungs, and odontogenic foci were predisposing factors in approximately 70% of cases. Single abscesses, present in 75% of patients, were distributed equally in both hemispheres, with more than half in the frontal and parietal lobes. Common signs and symptoms included headache, fever, chills, seizures, nausea, vomiting, altered sensorium, nuchal rigidity, and localizing neurologic signs. Blood cultures were positive in 11%. Lumbar puncture rarely provided data from which a diagnosis could be established; CSF cultures were positive in only 7% of patients, and there was a 15% temporally associated incidence of brain herniation and death. Diagnostic information was most readily obtained using imaging techniques such as CT and 99mTc scanning, and arteriography was invasive and of no added value. CT scans are however, often initially negative in patients presenting with clinical signs of meningitis presumably following rupture of an abscess into the subarachnoid space, and the average time for changes to appear on CT scan is 9 days. It is, therefore, recommended that when the clinical assessment suggests the possibility of brain abscess the patient be treated empirically with antibiotics and that lumbar puncture be performed only after thoughtful assessment of the risk-to-benefit ratio for each patient. Causative organisms were isolated from more than 80% of abscesses despite prior antibiotic treatment; more than half grew a single pathogen, most commonly streptococci. Anaerobic and microaerophilic bacteria accounted for 62% of all isolates, and were the only organisms in 33% of patients. Computerized tomographic scans in 30 patients showed "ring-enhancing" lesions, nodular enhancement, or areas of low attenuation. Complete resolution of abscesses on CT scans rarely occurred during hospitalization and took as long as 5 months. Decrease in the size of abscesses on CT scan correlated well with clinical improvement and was seen within a week when abscesses were excised, but was often not obvious for 6 to 8 weeks if antibiotics were used alone.(ABSTRACT TRUNCATED AT 400 WORDS)